Such field probes are typically used to measure RF fields for all manner of applications, such as measuring the electrical field strength during testing of radiation immunity, calibration of the electrical field in test arrangements, determining the homogeneity of the electrical field in a test installation or measuring the electrical field strength in order to determine whether the field levels lie below a value acceptable for persons.
The known field probes typically use a number of antenna elements and a circuit for converting the RF signal into a DC current with an amplitude which is a function of the amplitude of the RF signal. It is desirable that such probes operate properly, and in particular are isotropic in a wide frequency and amplitude range.
Such a probe is for instance described in U.S. Pat. No. 5,057,848. The probe described therein has three antenna sub-systems which are oriented along respectively an X, Y and Z axis. The antenna elements are connected to a detection/filter circuit, from where the detected signals are transmitted to a processing circuit connected to a power supply and communications circuit, from where optical data signals are sent via an optical fibre for further processing, for instance on a PC. Each antenna sub-system comprises a monopole 60 and a disk antenna 64. The monopole antenna is here connected to a HF detection circuit via a “splice or crimp 94-conductive portion 96-socket 98” structure. This HF detection circuit transmits the detected signals to a processing circuit which is arranged together with the power supply circuit on PCBs 396, 398 in the housing—see FIG. 12. Such a probe has the drawback that there is not a good separation between detection circuit on the one hand and processing circuit on the other, and that the detection diode is arranged in the housing. The probe will hereby be inaccurate in measurements of high-frequency weak signals.
Probes are also known which make use of dipole antenna elements with resistive parts, wherein the detection circuit is provided in the centre or on the side of the dipole antennas. U.S. Pat. No. 6,593,869 thus describes a probe with X, Y and Z dipole antennas, wherein each dipole comprises two twisted branches with discrete resistive elements and the diodes are arranged in the centre of the two branches. The article by Motohisa Kanda et al with the title “An Isotropic Electric-Field Probe with Tapered Resistive Dipoles for Broad-Band Use, 100 kHz to 18 GHz”, IEEE, Transactions on Microwave Theory and techniques, vol. MTT-35, no. 2, February 1987, further describes a probe with dipole antenna elements with tapered resistive elements. The most significant drawback of such antenna elements is that the field probe provides a non-isotropic response because of the connecting wires with high impedance which are necessary between the antenna elements and the processing circuit, wherein the processing circuit must be mounted away from the centre.